The present invention generally relates to masks for forming patterns on semiconductor devices and the like, mask producing methods and pattern forming methods using masks, and more particularly to a mask which uses light phase shift principles, a mask producing method for producing such a mask and a pattern forming method which uses such a mask.
When forming patterns of elements and circuits and the like on semiconductor wafers in production processes for semiconductor devices, it is normal to employ ultraviolet light for the pattern transfer exposure.
Patterns to be transferred onto a wafer are formed utilizing a thin, opaque metal film provided on a glass substrate which is transparent with respect to the exposure light. The pattern on the glass substrate to be transferred onto the wafer is called a mask when this pattern is identical to the chip pattern which is actually transferred onto the wafer. On the other hand, the pattern on the glass substrate is called an enlarged mask or a reticle when this pattern is enlarged compared to the chip pattern which is actually transferred onto the wafer. When using the mask, the pattern is transferred to a resist layer on the wafer using parallel ray exposure. On the other hand, when using the reticle, the pattern is transferred to the resist layer on the wafer using a reduction lens exposure system which projects a reduced pattern onto the resist layer.
In order to improve the resolution of the pattern which is transferred, especially when the integration density of the integrated circuit is large and the pattern is fine, it is necessary to improve the contrast at the edge portions of the region which is exposed.
FIG. 1 shows an example of a conventional mask which is made up of an opaque layer having a predetermined pattern and a transparent substrate. FIG. 2 shows an example of an optical system for forming a pattern on the wafer using the mask shown in FIG. 1.
In FIG. 1, a mask 450 comprises a transparent substrate 452 and an opaque layer 451 which is made of a material such as chromium (Cr). The opaque layer 451 is formed to a predetermined pattern by lithography and etching processes.
In FIG. 2, a light C which is emitted by an exposure apparatus (not shown) illuminates the mask 450. The light C cannot be transmitted through the opaque layer 451 but is transmitted through the portions of the transparent substrate 452 not provided with the opaque layer 451. The transmitted light passes through an imaging lens system 453 and is used to expose a resist material 455 which is coated on a wafer 454. For example, a resist OFPR manufactured by Tokyo Ooka Kogyo K. K. of Japan may be used as the resist material 455. As a result, a pattern identical to the pattern of the mask 450 is formed on the wafer 454 by an etching process.
When forming the pattern using an optical lens system, the exposure of wafer 454 is based solely on the contrast available as a result of the existence of the opaque layer 451 on the transparent substrate 452. For this reason, the resolution of the pattern formation is physically limited by the wavelength of the light which is obtained via the optical lens system, and depending on the wavelength of the light used for the exposure it is often difficult to form a fine pattern.
Conventionally, photolithography processes employ reticles made by forming an opaque layer on a transparent substrate and patterning the opaque layer. For example, the opaque layer is made of Cr and the transparent substrate is made of a transparent material such as glass and quartz. FIGS. 3A through 3D are diagrams for explaining such a conventional pattern forming method.
In FIG. 3A, the optical system includes a light source 461, an illumination lens 462, and an imaging lens system 465. A reticle 464 is arranged between the illumination lens 462 and the imaging lens system 465. The light source 461 may be a mercury lamp or an excimer laser or the like and is provided with a filter for emitting i-ray or g-ray beams. The light 463 from the light source 461 illuminates the reticle 464 via the illumination lens 462. For example, the partial coherency .sigma. of the illumination lens 462 is 0.5. The reticle comprises a transparent substrate 468 and an opaque pattern 469 which is formed on the transparent substrate 468. For example, the transparent substrate 468 is made of glass and the opaque pattern 469 is made of Cr. The opaque pattern 469 on the reticle 464 is imaged on a photoresist layer 467 formed on a semiconductor substrate 466 via the imaging lens system 465. For example, the imaging lens system 465 has a numerical aperture (NA) of 0.50.
According to this pattern forming method, the resolution is described by the expression K1.multidot..lambda./NA, where K1 denotes a process coefficient which is normally 0.6 to 0.8 and .lambda. denotes the wavelength of the light. The wavelength .lambda. of the light 463 is approximately 365 nm in the case of i-rays emitted using a mercury lamp and is 248 nm or 198 nm when an excimer laser is used. The numerical aperture NA differs depending on the imaging lens system 465 which is used, but is approximately 0.5, for example. In order to improve the resolution, it is necessary to set K1 or .lambda. to a small value and to set NA to a large value. However, the values of K1 and NA cannot be set freely. In addition, the value of .lambda. is restricted by the light source 461 and the optical system used. The resolution is determined when the wavelength .lambda. of the light used for the exposure, the numerical aperture NA and the process coefficient K1 are set, and it is impossible to image a pattern finer than the resolution.
The light 463 emitted from the light source 461 illuminates the entire surface of the reticle 464, and the portion of the light 463 which illuminates the opaque pattern 469 is blocked by the opaque pattern 469. For this reason, only the portion of the light 463 which illuminates the portion of the reticle 464 not provided with the opaque pattern 469 is transmitted through the reticle 464 and is imaged onto the photoresist layer 467 via the imaging lens system 465. FIG. 3B shows the electrical vector E of the light transmitted through the reticle 464, and FIG. 3C shows the light intensity P of the light transmitted through the reticle 464. A pattern having a light intensity distribution proportional to the square of the amplitude of the light illuminating the photoresist layer 467 is formed on the photoresist layer 467, and the photoresist layer 467 is selectively exposed.
FIG. 3D shows a portion of the reticle (opaque mask) 464 on an enlarged scale. The minimum width W of the pattern which can be exposed is dictated by the resolution which is a function of the imaging lens system 465.
FIGS. 4A through 4D illustrate the light intensity distribution for cases where the pattern which is exposed is a line pattern which is finer than the resolution. In FIGS. 4A through 4D, the wavelength .lambda. of the light used is 365 nm, the numerical aperture NA is 0.50 and the partial coherency .sigma. of the light is approximately 0.5.
FIG. 4A shows the light intensity distribution for a case where the line pattern which is imaged has a width of 0.35 .mu.m. The light intensity approaches zero approximately at the center position (0.0) and gradually rises on each side of the center position (0.0). The line width is approximately 1.0 .mu.m or greater at a position where the light intensity is at a maximum. When the photoresist layer is exposed using a position where the light intensity is approximately 0.2 as a developing threshold, it is possible to develop a pattern which has a design width in the order of 0.35 .mu.m.
FIG. 4B shows the light intensity distribution for a case where the line pattern which is imaged has a width of 0.30 .mu.m. The minimum value for the light intensity at the center position (0.0) is greater than the minimum value for the light intensity distribution shown in FIG. 4A. The width of the light intensity distribution curve itself as shown in FIG. 4B is not much different from that of FIG. 4A.
FIGS. 4C and 4D respectively show the light intensity distributions for cases where the line patterns which are imaged have widths of 0.25 .mu.m and 0.20 .mu.m. Similarly to the case of FIG. 4B, in each of FIGS. 4C and 4D the minimum value for the light intensity at the center position (0.0) is greater than the minimum value for the light intensity distribution shown in FIG. 4A. The width of the light intensity distribution curve itself as shown in each of FIGS. 4C and 4D is not much different from that of FIG. 4A. In other words, when the pattern width is reduced to a point which is less than the resolution, the width of the light intensity distribution curve which is obtained does not decrease and the minimum value for the light intensity at the center position (0.0) increases. In these cases it is possible to reduce the width of the line which is exposed, and a black area is exposed instead as a gray area. For these reasons, it is impossible to form an image which is finer than the resolution.
On the other hand, a method of shifting the phase of the light which is transmitted through the mask and exposed on the wafer by 180.degree. depending on the patterns of the mask has been proposed by Marc D. Levenson, "Improving Resolution in Photolithography with a Phase-Shifting Mask", IEEE TRANSACTIONS ON ELECTRON DEVICES, Vol. ED-29, No. 12, December 1982. According to this proposed method, interference between the patterns is eliminated so as to improve the contrast on the wafer and improve the resolution of the exposure apparatus.
However, it is difficult to apply this proposed method to masks and reticles having fine patterns, and there is a problem in that it is troublesome to generate pattern data peculiar to the phase shift pattern. For this reason, there is a demand for realization of a phase shift pattern which is easily applicable to masks and reticles having fine patterns and which does not substantially increase the number of processes involved in the generation of the pattern data.
The phase shift pattern of the phase-shifting mask which is used according to this proposed method is formed as follows. First, an auxiliary pattern having a width smaller than that of the design pattern is formed in the vicinity of the design pattern (white pattern) which is to be transformed onto the wafer. Secondly, a phase shifter is formed on the auxiliary pattern. The phase shifter is an organic pattern made of a resist or the like formed by coating, exposure and developing processes, or an inorganic pattern formed by chemical vapor deposition and lithography processes. For example, a description of a method of forming a phase-shifting mask which uses a negative resist pattern as the phase shifter is as follows, with reference to FIGS. 5A through 5D.
In FIG. 5A, an opaque layer 552 is formed on a glass substrate 551. An aperture pattern, that is, a design pattern 553 comprising a transmission region, and fine patterns 554A and 554B is formed in opaque layer 552 by lithography using electron beam exposure. For example, the design pattern 553 has a width in the order of 1.5 .mu.m, and the fine aperture patterns 554A and 554B have widths in the order of 0.5 .mu.m. The fine aperture patterns 554A and 554B are formed as auxiliary patterns in regions neighboring the design pattern and are separated therefrom by a distance in the order of 0.5 .mu.m, for example.
Next, as shown in FIG. 5B, a transparent conductor layer 555 for preventing charge up at the time of the electron beam exposure is formed on the surface of the glass substrate 551 and on the opaque layer 552 so as to cover the inner sides of the aperture patterns 553, 554A and 554B.
Then, as shown in FIG. 5C, a negative resist layer 656 is formed on top of the entire assemblage to a thickness such that the phase of the light which is transmitted through the negative resist layer 656 is shifted by 180.degree.. A prebaking process is performed if needed, and a phase shift pattern is formed over the fine patterns 554A and 554B by electron beam exposure.
The thickness D of the negative resist layer 656 may be determined using the following formula (1), where .lambda. denotes a wavelength of the light used for the exposure and n denotes the refractive index of the shifter material. EQU D=.lambda./2(n-1) (1)
When using an i-ray beam having a wavelength of 365 nm for the exposure, the refractive index n of the negative resist layer 656 is approximately 1.6 and the thickness D becomes approximately 304 .mu.m.
Next, as shown in FIG. 5D, a developing process is performed to selectively form phase shift patterns 556A and 556B on the fine patterns 554A and 554B, respectively. The phase shift patterns 556A and 556B are made up of the negative resist layer 656 which has a thickness D.
FIG. 6 shows a phase profile corresponding to the phase pattern of an i-ray beam transmitted through the phase-shifting mask of FIG. 5D when an i-ray beam is used for the exposure.
On the other hand, when a positive resist is used, a phase shift pattern 557 which is made up of the positive resist is selectively formed on the design pattern 553 as shown in FIG. 7 by processes similar to those described in conjunction with FIGS. 5A through 5D. In FIG. 7, those parts which are essentially the same as corresponding parts of FIGS. 5A through 5D are designated by the same reference numerals, and a description thereof will be omitted.
FIG. 8 shows a phase profile corresponding to the phase pattern of an i-ray beam transmitted through the phase-shift mask of FIG. 7 when an i-ray beam is used for the exposure.
When the phase-shifting masks of FIGS. 5D and 7 are used, the phase of the portions of the i-ray beam (i.sub.a and i.sub.c) transmitted through the design pattern 553 differs by 180.degree. from the phase of the portions of the i-ray beam (i.sub.b and i.sub.d) transmitted through the fine patterns (auxiliary patterns) 554A and 554B, as may be seen from FIGS. 6 and 8. For this reason, i-ray beams scattering in the horizontal direction from the region immediately below the design pattern (i.sub.a and i.sub.c) is cancelled by i-ray beam scattering in the horizontal direction from the region immediately below the auxiliary pattern (i.sub.b and i.sub.d), and the contrast at the ends of the exposure region is improved thereby improving the resolution.
The aperture width of the auxiliary patterns 554A and 554B is made to be sufficiently narrow so that the light transmitted therethrough during a standard exposure is insufficient to expose the resist layer all the way to the bottom portions thereof. Hence, the auxiliary pattern will not be transferred onto the wafer when an exposure is made using the phase-shifting mask.
The methods described in conjunction with FIGS. 5 through 8 are proposed in Japanese Laid-Open Patent Applications No. 61-292643, No. 62-67514 and No. 62-18946.
However, conventional masks suffer from the following disadvantages.
First, in the case of masks which do not use phase shifting principles, it is difficult to form a pattern which is narrower than the wavelength of the light due to the physical resolution limits of the optical system. When an attempt is made to realize narrow line widths, it is necessary to make structural modifications such as by reducing the wavelength of the light or by increasing the numerical aperture. Accordingly, it is impossible to form fine patterns such as are desirable for future integrated circuits using an optical method.
Second, in the case of prior art masks which employ phase shifting, such pattern formation can only be applied to patterns such as so-called line-and-space patterns which have regularity and cannot be applied in the production of integrated circuits having various patterns. In addition, it is impossible to form a fine pattern because an opaque layer must always be provided.
Third, in the case of prior masks which use phase shifting, the auxiliary pattern must be patterned to a finer degree than the design pattern. For this reason, the fineness of the design pattern must be restricted so that the resolution limit of the auxiliary pattern is not exceeded.
Fourth, in the case of such masks which use phase shifting, the number of steps required to generate the pattern data is large because, in addition to the data related to the design pattern, it is necessary to generate the pattern data related to the auxiliary pattern, the pattern data related to the phase shift and the like.
Fifth, in the case of prior masks which utilize phase shifting, it is difficult to control the quality and thickness of the phase shifter material and this affects the refractive index when an organic resist material is used as the phase shifter. For this reason, it is difficult to form a uniform phase shift pattern having accurate phase shift qualities.
Sixth, in the case of prior masks which employ phase shifting, the phase shifter is made of a material that is different than the glass substrate. Thus, a reflection occurs at the boundary of the phase shifter and the glass substrate to thereby deteriorate the exposure efficiency.